Why is the boolean modifier creating a glitchy mesh instead of cutting into the object?
If you want to cut something into an object using boolean, the modifier has to be set on the mesh that you want to edit. Not on the stencil object. You also may change the visibility of the scencil object from textured to bounds.

It then should just look like this 

Also i recommend using the BoolTool Addon. It is far more user friendly.